The present invention relates to a method for machining a shaft or cylindrical article such as a roll.
Rolls are used in various rolling-mill stands for rolling workpieces. Use of a roll at a temperature which is different from room or ordinary temperature may cause roundness of the roll to be degraded by deviations in thermal expansion or shrinkage of the roll, i.e., by distortions of the roll so that axial bending of the roll may occur within the limits of elastic deformation of the roll. Such bending will result in so-called runout of the roll during its rotation. For example, when calendering is carried out with such runout occurring on calender rolls, a sheet a thus rolled has surface irregularities in a longitudinal direction thereof as shown in FIG. 6 so that only a portion b of the sheet a can be used as calendered product in practice, resulting in a lower degree of accuracy in thickness and poor yield as well as lack of accuracy in patterns and color tones.
In order to prevent the above-mentioned distortions, it has been proposed to machine a roll at a working temperature of the roll.
However, machining of a roll at its working temperature is frequently found to be unpractical especially when the working temperature is high (for instance, a working temperature in calendering a sheet of polyvinyl chloride is about 180.degree.-220.degree. C.) from standpoints of how to heat a roll, how to maintain the elevated temperature, uniformity of heat distribution (thermal non-uniformity, easiness and safety in operation and thermal effects on machinery used. As a consequence, rolls having a sufficient degree of machining accuracy at room temperature are generally used even in rolling at higher working temperatures. It has been also practiced to set an accuracy reference and utilize rolls having a poor accuracy below the accuracy reference to applications other than rolling. As a result, problems of finished sheets having lower qualities and lower yields still remain.
In view of the above, a primary object of the present invention is to provide a machining method by which a shaft is made to have substantially perfect roundness in cross section under actual working conditions.